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Following the outbreak of the H1N1 influenza pandemic of 1918, which came to be known as the Spanish Flu, the number of deaths due to influenza and pneumonia soared. Pneumonia was caused either by the influenza or by a bacterial superinfection that took hold due to the patient's weakened state as a result of the influenza, for this reason, influenza deaths and pneumonia deaths were recorded together as one. Pennsylvania had the highest mortality rate due to the pandemic, where there were over 880 fatalities per 100,000 people; meaning that approximately 0.9 percent of the state's population died from the Spanish Flu pandemic in 1918.

When compared with the 1915 mortality rates, many states, such as California and Pennsylvania, saw their mortality rate due to influenza and pneumonia increase five-fold by 1818, which was the worst year of the pandemic. While the mortality rate decreased significantly in the year 1919, there was no US state where it fell to it's pre-pandemic level, and the 1919 mortality rate was still double the pre-pandemic rate in some states such as California, South Carolina and Washington.